This is the format. Why PDF? Because readers want stability. Unlike a web page that can be deleted or a streaming service that can lose licensing, a PDF is a file. It lives on your hard drive, tablet, or e-reader. It is offline-friendly, searchable, and permanent. For fans of niche or controversial genres, PDFs represent a hedge against censorship or link rot.
Search for "forbidden" classics. The Monk by Matthew Lewis, Justine by the Marquis de Sade (historically significant, if extreme), or Venus in Furs by Leopold von Sacher-Masoch. These are public domain, completely legal, and available in pristine PDFs.
For classic "taboo" literature (like Flowers in the Attic by V.C. Andrews or Tropic of Cancer by Henry Miller), the Internet Archive offers legal scans. The quality varies, but you can often find "better" editions by filtering by "PDF" and "Borrow."